Brochure/Catalogue
Components for Surge protection
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2. Varistors / MOV
The varistors used with surge protection (MOV-Metal Oxide
Varistors) have resistance which depends on the voltage.
This is implemented with metal-oxide (zinc-oxide) discs.
There is a low-ohm resistance in the range above the rated
voltage. The surge voltage is limited since a current ows
through the varistor. The varistor works bi-directionally.
Depending on the type, varistors have either a middle or high
discharging capacity. It is in the range from 40 kA to 80 kA.
The response time is less than 25 ns. However there are also
disadvantages when using varistors. Two factors that must
be taken into account are the relatively high capacitance and
the aging characteristics.
Leakage currents occur over time, depending on the
frequency of the triggering, because individual resistance
elements break down. This can cause temperature rise or
even destroy them completely.
This is one reason for thermal fuses being built into
Weidmüller products. The high capacity of the varistors is
problematic for circuits with high frequencies. Some signal
attenuation should be expected at frequencies above 100
kHz. We therefore recommend that they are not used in data
transmission systems.
3. Suppression diodes / TAZ
Suppressor diodes function in a similar fashion as
Zenerdiodes. There are uni-directional and bi-directional
versions. Uni-directional suppressor diodes are often
used in DC circuits. Compared to standard Zener diodes,
suppressor diodes have a higher current-carrying capacity
and are signicantly quicker. At a certain breakdown voltage
level, they become conductive very quickly. They therefore
discharge the surge voltage. However their current-carrying
capacity is not very high. It is only a few hundred amps.
Instead, they feature a very quick reaction time which lies in
the picosecond range.
Unfortunately, suppression diodes possess a signicant
inherent capacitance. Therefore, like with varistors, their
possible attenuation effect on high frequencies must be
taken into account.
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The basics of lightning and surge protection
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